Measurement of fatigue accumulation in high-strength steels by microstructural examination
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INTRODUCTION
FATIGUE damage accumulation that results in cracking or fracture has long been recognized as a major cause of failure of engineered structures. Many design rules have been established to insure that fatigue levels are held below those that would permit failure. These rules, such as those of the ASME Boiler and Pressure Vessel Code, Section III for nuclear plant pressure boundary components, are quite effective whenever it is possible to identify or bound all transients that contribute to fatigue in operations. Inspection becomes of interest when such transient identification is not possible, the objective being to validate that damage accumulation is less than that which will permit rapid deterioration to a failure state. In-service measurement of fatigue damage is creditably successful when creep is present since the associated microvoids and their growth patterns can be detected by available techniques such as surface replication. Less success is apparent for structures made from materials operating below the creep condition. One reason is that the precrack microstructural effects of damage are not well identified; a second is that the signal-to-noise ratio of expected effects is probably small with respect to the random variation noise level present in typical polycrystalline engineering materials. The objective of the present study is to identify an effective microstructural method of measuring the fatigue damage accumulation state of typical nuclear plant pressure vessel steel operating in the elastic range. Potential techniques for the damage assessment are
Y.G. NAKAGAWA, Chief Research Engineer, and H. YOSHIZAWA, Research Engineer, are with the Research Laboratories, Ishikawajima-Harima Heavy Industries Company, Ltd., Tokyo, Japan. M.E. LAPIDES, Technical Specialist, is with the Electric Power Research Institute, Palo Alto, CA 94304. Manuscript submitted September 13, 1989. METALLURGICAL TRANSACTIONS A
positron annihilation,tl] X-ray diffraction,t2'3] magnetic technique, t4] acoustic emission, ts,6] ultrasonic technique, t7,sl and other miscellaneous methods. [9] In general review,t9] these have been found effective to detect cracks and voids but ineffective for detection of microstructural changes associated with fatigue, except perhaps for singlecrystal materials. X-ray diffraction line-broadening (XRD) has met limited success. The broadness of the diffraction line is related to the microstructure of polycrystalline materials. The shape of the diffraction line, which is characterized by the half-value breadth (full width at half maximum peak height), is influenced by the combined effects of various microscopic parameters, such as microlattice strains, subgrain size, dislocation density, and the micro-orientation of individual crystals. It has been recognized, however, that the changes in the XRD could be found in the initial portion of fatigue life due to the introduction of cold work and to the terminal stage near the cracking or rupture. [1~ A linear relationship between an XRD measur
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